Designing and Building
File-Folder Bridges
A Problem-Based Introduction to Engineering
“Every bridge begins in the mind of an engineer”
Our Purpose
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design, build, and test model bridges;
use an authentic engineering design process
to develop designs;
apply math, science, and computer
technology as problem-solving tools;
learn how real bridges are designed and built;
and
learn how real truss bridges work.
Overview of the Activities
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Activity #1: Build a model of a truss
bridge.
Activity #2: Test the strength of structural
members.
Activity #3: Analyze and evaluate a truss.
Activity #4: Design a truss bridge with a
computer.
Activity #5: Design and build a model
truss bridge.
Activity #1- Build a model of a truss bridge.
In this activity, we will build a bridge with an existing design so that:
 We can learn about many key concepts about trusses and
structural behavior that you’ll use when you design your own
bridge in Activity #5.
 Familiarize with the engineering characteristics of a rather unique
building material—cardboard from a manila file folder.
 Learn some special construction techniques appropriate for this
material.
 Work with confidence, knowing that your bridge will carry the
prescribed loading successfully, as long as you build the
structure with care.
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Learn about the challenges faced by real-world construction
contractors, who are often required to build structures that have
been designed by someone else.
Activity #1 Goals
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Explain what a truss is.
Identify the major components of a truss bridge.
Identify the types of truss bridges.
Explain the following fundamental structural engineering
concepts: force, load, reaction, equilibrium, tension,
compression, and strength.
Explain how a truss bridge works—how each individual
component contributes to the ability of the entire structure to
carry a load.
Explain the roles of the four key players in the designconstruction process—the Owner, the Design
Professional, the Constructor, and the Project Manager.
Explain how construction quality affects the performance of a
structure.
Component Parts of a Truss Bridge
What is a Truss?
A truss is a structure composed of members connected
together to form a rigid framework. Members are the
load-carrying components of a structure. In most trusses,
members are arranged in interconnected triangles, as
shown below.
Component Parts of a Truss Bridge
What is a Truss?
Because of this configuration, truss members carry load primarily in
tension and compression. Because trusses are very strong for their
weight, they are often used to span long distances. They have been
used extensively in bridges since the early 19th century; however, truss
bridges have become somewhat less common in recent years. Today
trusses are often used in the roofs of buildings and stadiums, in
towers, construction cranes, and many similar structures and
machines.
Component Parts - Elevation
View
Component Parts - Isometric
View
Types of Truss Members
Connecting the Members:
Pinned and Gusset Plate
Pinned connections were used extensively throughout the 19th
century. Most modern bridges—including the model bridge we will
be building here—use gusset plate connections.
The Foundation
Bridges use two different types of foundations. The ends of a
bridge usually rest on abutments, which serve two functions
simultaneously—they support the bridge and also hold back
the soil that is filled in behind them. If the bridge requires
additional support in the middle of the gap, one or more piers
are used, as shown below. Abutments and piers are normally
made of concrete.
Types of Truss Bridges
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If the deck is located at
the level of the bottom
chord, the bridge is called
a through truss.
A pony truss looks just
like a through truss,
except it is not as high
and has no lateral bracing
between the top chords.
If the deck is located at
the level of the top chord,
the bridge is called a deck
truss.
15 of the most common truss
configurations
The Problem
The Need
Just outside the small town of Hauptville,
New York, Grant Road crosses Union
Creek via a beautiful old 19th Century Pratt
truss bridge similar to the one shown here.
Recently, the Town Engineer determined
that the structure is no longer safe for
modern truck traffic and must be replaced.
Because of its historic value, the old bridge
will be disassembled, moved to a nearby
public park, and rebuilt as a pedestrian
bridge. A new highway bridge for Grant
Road must be built on the existing site.
Design Requirements
The Owner for this project is the Town of Hauptville. Several
months ago, the Town Council selected Thayer Associates, a
respected local engineering firm, as the Design Professional for
this project. The Hauptville Town Engineer worked closely with
civil engineers from Thayer Associates to develop three
functional requirements for the bridge:
• The new bridge must be constructed on the abutments from
the old structure.
• These existing supports are 24 meters apart. [Our 1/40 scale
model bridge will actually have a span of 60 centimeters.]
• The bridge must carry two lanes of traffic. [Our model bridge
must have a roadway width of at least 9 centimeters and at
least 9 centimeters of overhead clearance above the deck]
Design Requirements
The bridge must meet the structural safety requirements of the AASHTO
(American Association of State Highway and Transportation Officials,)
bridge design code.6
[Our model bridge must carry a “traffic load” consisting of a 5 kilogram
mass placed on the structure at mid-span.]
The Town Council also added an important aesthetic requirement. To
preserve the town’s historical character, the new Grant Road Bridge
should look similar to the old one— a Pratt through truss. The old
bridge was made of wrought iron, but the Town Engineer has decided
that the new structure will be safer and more practical if it is made of
steel.
[For our model, steel will be represented by cardboard from manila file
folders.]
The Design
Based on these design requirements, a team of
engineers from Thayer Associates has
developed plans and specifications for the new
Grant Road Bridge over Union Creek. The
plans and specifications include a structural
drawing, isometric drawings of two typical
connections, a schedule of truss members, a
schedule of connections, and full-scale shop
drawings of the structure.
Structural Drawing
The structural drawing of the new Grant Road
Bridge is designated as Drawing S-1 and is
provided. The drawing includes a side
elevation, a front elevation, and a plan view.
Note that every connection in the structure
is designated with a letter—A through N for
one main truss and A’ through N’ for the
other. These letters are used to identify the
members and gusset plates.
Typical Connections
The two isometric drawings below are typical gusset-plate
connections found at the top and bottom chords of the main
trusses.
These drawings illustrate the types of structural members used throughout
the Grant Road bridge—hollow tubes for the top chords and verticals;
doubled bars for the bottom chords and diagonals. The drawings also show
how two gusset plates are used at each connection to hold all of the
structural members together.
Schedule of Truss Members
The Schedule of Truss Members identifies every member required to build
the bridge. Note that each member is identified by the two letters
corresponding to its endpoints. For example, Member AD is a
segment of the bottom chord that goes from Connection A to
Connection D.
The Plan
Congratulations on your selection as the Constructor for the Grant
Road Bridge project! You have received the plans and
specifications, and the Owner has given you the notice to
proceed—an official authorization to start work on the project.
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Obtain the necessary supplies and tools.
Prefabricate the structural members and connections.
Set up the construction site.
Build the structure.
Perform a quality control inspection.
Put the bridge into service.
Materials
To build the Grant Road Bridge, you will need the supplies and tools
shown below. In addition, you’ll need the full-size bridge plans
that were included.
 3 standard manila file folders
 A building board made of cork or soft wood, measuring at least 45cm
by 60cm.
 Wax paper
 Pins
 Small hammer
 Sharp pair of scissors
 Sharp hobby knife or single-edge razor blade
 Metal ruler or wooden ruler with a metal edge
 Ball-point pen
 Yellow wood glue
 Rubber cement
Prefabricate the Bars
See Construction Notebook!